The present invention relates to monitoring during chemical mechanical polishing.
An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive or insulating layers on a silicon wafer. One fabrication step involves depositing a filler layer over a non-planar surface, and planarizing the filler layer until the non-planar surface is exposed. For example, a conductive filler layer can be deposited on a patterned insulating layer to fill the trenches or holes in the insulating layer. The filler layer is then polished until the raised pattern of the insulating layer is exposed. After planarization, the portions of the conductive layer remaining between the raised pattern of the insulating layer form vias, plugs and lines that provide conductive paths between thin film circuits on the substrate. In addition, planarization is needed to planarize the substrate surface for photolithography.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing disk pad or belt pad. The polishing pad can be either a “standard” pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
An important step in CMP is detecting whether the polishing process is complete, i.e., whether a substrate layer has been planarized to a desired flatness or thickness, or when a desired amount of material has been removed. Overpolishing (removing too much) of a conductive layer or film leads to increased circuit resistance. On the other hand, underpolishing (removing too little) of a conductive layer leads to electrical shorting. Variations in the initial thickness of the substrate layer, the slurry composition, the polishing pad condition, the relative speed between the polishing pad and the substrate, and the load on the substrate can cause variations in the material removal rate. These variations cause variations in the time needed to reach the polishing endpoint. Therefore, the polishing endpoint cannot be determined merely as a function of polishing time.
To detect the polishing endpoint, the substrate can be removed from the polishing surface and transferred to a metrology station. At the metrology station, the thickness of a substrate layer can be measured, e.g., with a profilometer or a resistivity measurement. If the polishing endpoint is not reached, the substrate can be reloaded into the CMP apparatus for further processing.
Alternatively, polishing can be monitored in situ, i.e., without removing the substrate from the polishing pad. In-situ monitoring has been implemented with optical and capacitance sensors. For in-situ endpoint detection, other techniques propose monitoring friction, motor current, slurry chemistry, acoustics, or conductivity. A recently developed endpoint detection technique uses eddy currents. The technique involves inducing an eddy current in the metal layer covering the substrate, and measuring the change in the eddy current as the metal layer is removed by polishing.